Generally, in the DRAM processes, the technology of field oxide isolation is used to form the LOCOS structure (as shown in FIG. 1) on a substrate. FIG. 1 shows a cross-sectional view of a stage in the conventional fabrication of the LOCOS structure. A field oxide layer 12 is formed on a substrate 10 and a pad oxide layer 13 is formed between the field oxide layer 12 on the substrate 10. The field oxide layer is a silicon oxide layer. There is the stress existing between the edge of the field oxide layer and the substrate. The smaller IC scale the more stress. This stress is generated from the different size of silicon lattices and silicon oxide lattices. Furthermore, after forming the field oxide layer, the sequential step is a etchback step. The etchback step is to remove the pad oxide layer and bird's beak and to define the active area. FIG. 2 shows a cross-sectional view of completing the etchback step in the conventional fabrication of the LOCOS structure. This etchback step is done by a wet etching process with 10% HF solution. Usually, the wet etching process has an over-etching to ensure removal of bird's beak. This results in the field oxide edge with the steep angle of about eighty to ninety degrees. The steep angle will make the stress on the bird's beak stronger, indicated by arrows 27. The stronger stress on the bird's beak, the easier to appear the dislocation phenomenon on the substrate. Moreover, the stress will cause the stress-induced defect in LOCOS structure, indicated by an arrow 28, because the sequent processes, such as the plasma etching or the ion implantation, damage the lattice resulted in releasing the energy of the lattice when the heating process. This defect, indicated by an arrow 28, will cause the current leakage from the junction of memory cell in the DRAM. The total leakage current of the cell must be low enough that the cell does not discharge and lose its memory state between refreshes. Thus, the refresh time of the DRAM will reduce or the retentive memory of the DRAM will discharge resulted from the current leakage. It makes the performance of the DRAM worse.
In addition, the stress of the field oxide edge will retard the next step, the gate oxide layer growing, so as to further lead to the gate-oxide-thinning effect. The gate-oxide-thinning effect is that the gate oxide layer can not grow on the bird's beak of field oxide layer as shown in FIG. 3. This thinning occurs even after a sacrificial oxide process, due to the large oxide stress. From the FIG. 3, a field oxide layer 12 is formed on the substrate 10 and then a gate oxide layer 13 is formed on the field oxide layer 12. The phenomenon of the thinning effect is indicated by the arrow 29. This field-oxide-thinning effect can produce the problem with respect to field-edge leakage. Finally, it also results in shortening the refresh time of the DRAM and makes the performance of the DRAM worse.